[PATCH] erofs-utils: mkfs: bound-check s3 passwd_file credentials

[Vansh Choudhary <ch@vnsh.in>]

mkfs_parse_s3_cfg_passwd() only checked the total passwd_file size,
which left two issues in the parser:

- a file exactly as large as the temporary buffer left no room for the
  trailing NUL byte;
- either credential could still exceed its destination buffer after the
  string is split at ':'.

Use sizeof(buf) for the temporary buffer check and reject overlong
access key or secret key fields before copying them out.

This keeps the existing parsing flow intact while making the bounds
checks match the actual destination sizes.

Signed-off-by: Vansh Choudhary <ch@vnsh.in>
---
 mkfs/main.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mkfs/main.c b/mkfs/main.c
index 58c18f9..eb13aba 100644
--- a/mkfs/main.c
+++ b/mkfs/main.c
@@ -663,7 +663,7 @@ static int mkfs_parse_s3_cfg_passwd(const char *filepath, char *ak, char *sk)
 		erofs_warn("passwd_file %s should not be accessible by group or others",
 			   filepath);
 
-	if (st.st_size > S3_ACCESS_KEY_LEN + S3_SECRET_KEY_LEN + 3) {
+	if (st.st_size >= sizeof(buf)) {
 		erofs_err("passwd_file %s is too large (size: %llu)", filepath,
 			  st.st_size | 0ULL);
 		ret = -EINVAL;
@@ -687,6 +687,12 @@ static int mkfs_parse_s3_cfg_passwd(const char *filepath, char *ak, char *sk)
 	}
 	*colon = '\0';
 
+	if (strlen(buf) > S3_ACCESS_KEY_LEN ||
+	    strlen(colon + 1) > S3_SECRET_KEY_LEN) {
+		ret = -EINVAL;
+		goto err;
+	}
+
 	strcpy(ak, buf);
 	strcpy(sk, colon + 1);
 
-- 
2.43.0